The present invention relates to a method for making optical waveguide preforms from which optical waveguide fibers are drawn.
There are several well-known methods for making preforms from which optical waveguide fiber is drawn. These include outside vapor deposition (OVD), modified chemical vapor deposition (MCVD), vapor axial deposition (VAD), and plasma-enhanced chemical vapor deposition (PECVD).
Each of the above methods conventionally involve: i) delivery of a vapor flow containing glass forming precursors to an oxidation site such as, for example, the flame of a gas/oxygen burner or hot plasma zone adjacent to a deposition substrate or inside a deposition tube; ii) oxidation of the vapor flow to form a particulate or soot oxidation product; and, iii) collection of the particulate or soot oxidation product on the substrate or tube to form a preform. (In the PECVD process, the glass is deposited directly from the vapor phase onto the tube without the intermediate soot formation step.) The resulting soot preform, formed by the OVD and VAD methods, is then further processed, by sintering, to form clear glass from which an optical waveguide fiber is drawn. Preforms produced by MCVD and PECVD processes are generally clear after the deposition stage and can be drawn into fiber without a sintering step. Dopants may be included in the vapor flow to modify various characteristics of the resulting glass such as refractive index or coefficient of thermal expansion.
SiO.sub.2 -based optical waveguide fibers have long been commercially preferred. By providing a preform with a radially-varying refractive index profile, an optical waveguide fiber with the requisite waveguiding characteristics can be drawn therefrom. In order to provide the appropriate waveguiding characteristics, SiO.sub.2 has been doped with various compounds to alter its refractive index. These compounds include, for example, GeO.sub.2, TiO.sub.2, and P.sub.2 O.sub.5. Vapors containing these compounds are conventionally provided using metal halides such as GeCl.sub.4, TiCl.sub.4, and POCl.sub.3. See, for example, Blankenship U.S. Pat. No. 4,314,837 (Blankenship '837).
It is also known that certain compounds, such as rare earth elements, can be incorporated into the glass structure to provide other optical-functions including lasing and signal amplification. See, for example, DiGiovanni et al., European Patent Application No. 0,469,795. It is also known that compounds containing these elements in readily vaporizable form are available, including the .beta.-diketonate complexes. See, for example, Miller et al. U.S. Pat. No. 4,501,602. An exemplary material for supplying ErO.sub.2 precursors is erbium heptafluoromethyloctanedione, Er(fod).sub.3.
Other methods of fabricating preforms containing rare earth dopants include sol-gel (see, for example, DiGiovanni et al. U.S. Pat. No. 5,123,940) and solution doping (see, for example, Ainslie et al. U.S. Pat. No. 4,923,279). These methods involve extra processing steps and provide poor control of the concentration of rare earth dopants in the resulting preform.
Prior methods of delivering the vapor flow to the oxidation site have included mixing the SiO.sub.2, precursors with the dopant precursors before oxidation occurs. See, for example, Mansfield et al. U.S. Pat. No. 4,826,288 (MCVD process wherein the vapors containing rare earth compounds, aluminum compounds, and glass forming elements are mixed just prior to entry into the reaction-, i.e., oxidation-, zone); Tumminelli U.S. Pat. No. 5,141,549 (method for planar optical waveguide fabrication wherein vapors containing rare earth compounds, aluminum compounds and SiO.sub.2 compounds are mixed at an oxidation burner to produce a uniform distribution of the rare earth and aluminum elements throughout the SiO.sub.2 soot).
Powers U.S. Pat. No. 4,639,079 discloses a VAD method for producing an optical fiber preform in which the core region is laid down in conically shaped layers. The conically shaped layers comprise two sublayers. One sublayer has a high dopant concentration and the other sublayer has a low, or no, dopant concentration. These sublayers are produced by multiple burners which are traversing one portion of the preform so produced. Each burner contains at least SiO.sub.2 precursors and also contains dopant precursors (for example, GeO.sub.2) at different levels.
We have found, however, that the vapors containing rare earth compounds formed from .beta.-diketonate complexes can be chemically incompatible with vapors formed from metal halides. Mixing of these chemically incompatible vapors prior to oxidation allows reactions to occur in the vapor stream. These vapor phase reactions can result in the formation of unwanted particulates in the vapor stream which can cause non-uniform deposition of the oxidation soot products as well as equipment problems such as plugging of burner orifices and contamination of subsequently manufactured preforms.